Water contaminated with metals and toxic organics represents a significant threat to human health and safety and to the quality of water supplies and aquatic environments. Clean-up of contaminated groundwater and industrial waste water is very costly. Heavy metal contamination of water arises from a variety of sources including acid mine drainage, industrial activities which generate contaminated waste water, such as electroplating or electrowinning processes, and remediation activities at toxic waste sites. Chromium, lead, copper, zinc and mercury are commonly-reported heavy metal contaminants with nitrate and sulfate being the most commonly reported anion. Heavy metals frequently pose a long-term environmental hazard because they cannot be degraded or readily detoxified. Removal of trace levels of dissolved metals, i.e., levels below about 2000-4000 ppm, to achieve metal levels below about 100 ppm (preferably below 1 ppm) is a particularly difficult and costly problem.
The most common method currently applied to removal of heavy metal ions is chemical precipitation by adding base to the aqueous waste stream. Intrinsic disadvantages of this approach are high costs, large space requirements, multi-step processing, and the production of heavy wet sludge that itself requires disposal. Similar disadvantages apply to other methods, e.g., reverse osmosis, ion exchange, filtration and foam separation, under investigation for removal of trace metals.
Heavy metals can be removed from aqueous media using electrochemical processing to effect reduction of metal ions and deposition of the metals at the cathode: EQU M.sup.n+ +ne.sup.-.fwdarw.M
where the preferred anodic reaction corresponds to: EQU H.sub.2 O.fwdarw.1/2O.sub.2 +2H.sup.+ +2e.sup.-.
At high cathodic overpotentials in aqueous solution a parasitic reaction generating hydrogen: EQU 2H++2e.sup.-.fwdarw.H.sub.2
can occur decreasing the efficiency of the process. The use of conventional electrode configurations, plate and frame electrodes, packed-bed electrodes or static fluidized-bed electrodes, however, have a number of disadvantages. Frame and plate electrodes operating at moderate current densities require larger electrode surfaces and generally require higher operating voltages to achieve desired deposition resulting in high capital and energy costs. Packed-bed electrodes do not accommodate particle growth that occurs on metal deposition leading to interelectrode shorting. Static fluidized-bed electrodes can operate at much higher geometric current density, but inconsistent electrically contact with current collectors can result in low electrochemical efficiency and current collector instability.
Contaminated groundwater and industrial waste water containing toxic organics including aliphatic and aromatic hydrocarbons, halogenated (particularly chlorinated) hydrocarbons, and phenols also represent significant and costly clean-up problems. Again removal of trace levels (about 1000 ppm or less) of toxic organics from water is particularly difficult. Electrochemical processes in which the organic contaminant is oxidized at the anode to decompose the organic and ultimately generate CO.sub.2 can be used to remediate contaminated waste water. In this case, the typical concomitant cathode reaction is hydrogen generation. Removal of trace levels of organics using conventional electrode configurations, however, suffers from similar disadvantages including: a low overpotential for oxygen evolution (a competing reaction) and the formation of polymeric films on the electrode surfaces.
The present invention relates to the use of an advanced electrolytic cell technology employing a dynamic spouted electrode for efficient and cost effective removal of trace contaminants (metal ions and organics) commonly found in aqueous waste streams. The spouted electrode cell design has distinct advantages over conventional electrolytic cell technology which include i) lowering mass transfer limitations for trace contaminant remediation, ii) achieving high interelectrode geometric current densities because of the high surface area electrode, iii) cell design which allows convenient removal of electrode particles without cell disassembly, and iv) minimal opportunity for dendritic growth and interelectrode shorting.
The electrode cell design of this invention can utilize readily available, inexpensive materials for fabrication which result in significantly lower capital costs compared to conventional commercial electrolytic hardware. High surface area electrode particles facilitate the efficient removal of trace heavy metals and trace organics and facilitate minimizing electrolytic power consumption.
In particular for heavy metal deposition, the use of high surface area charged metal particles, which in the spouted cathode are in constant motion, avoids dendritic growth and agglomeration of cathode materials as metals are deposited from solution. The spouted cathode cell design of this invention provides the selective, efficient removal of trace heavy metal contaminants from common sources containing mixture of metal ions.
Previous work with spouted cathodes has focused on using the spouted-bed cathode for electrowinning of metals (Salas-Morales, J. C. et al. (1997)Metall.Mater. Trans. B 28B:59; Verma, A. et al. (1997) Metall. Mater. Trans. B 28B:69). U.S. Pat. No. 5,635,051 of Evans reports the use of a spouted cathode for zinc electrowinning. Electrowinning typically employs metal ion solution concentrations in the molar range, very much higher than present in the contaminated aqueous media of this invention. K. B. Mathur and N. Epstein, Spouted Beds Academic Press, N.Y., N.Y. 1974 provides general information on spouted bed electrodes.